The present invention relates to a method for machining and forming a transcurrent opening in a fibre-reinforced composite material starting from a preformed hole, in conjunction with which at least one rotatable grinding tool with a wear-resistant outer working surface and with a diameter considerably smaller than that of the preformed hole is positioned in the hole, and the hole is machined and formed by causing the grinding tool on the one hand to rotate about its own axis and on the other hand to describe a translation motion relative to the edge of the hole.
Polymer composite materials have been known since the 1950s. These materials are composed of a protective and consolidating polymer, either thermoplastic or thermosetting resin plastic, usually referred to as the matrix, and fibres (e.g. glass, carbon or aramide fibres), which may be regarded as reinforcing material. The fibres may be continuous and oriented in certain directions, or may be relatively short and arranged at random in the matrix. Composites with continuous and oriented fibres give products with superior mechanical characteristics to conventional polymer and metallic materials, in particular with regard to their weight-related strength and rigidity. Composites with shorter fibres find applications where their characteristics are subject to rather lower requirements. One inhibiting factor for the increased use of composite materials is the absence of effective methods of cutting machining. The physical and chemical properties of composite materials mean that familiar machining methods cannot be applied generally with successful results.
Composite materials for practical applications often contain holes for various purposes. The holes may be required, for example, to accommodate wiring or to permit assembly or inspection. One particularly important category of hole is bolt holes. Structures for practical applications are often built up from components which have been joined together to produce a finished product. The purpose of the joint is to transmit the load from one part of the structure to another. A common method of joining is the bolted joint in which the load is transmitted via either shearing forces or tensile forces in the bolt. The strength of a bolted joint is influenced to a considerable degree by the quality and accuracy of the holes. Reference can be made here to three particular problem areas which can arise in conjunction with making holes in polymer composite materials:
Low Interlaminar Strength.
When machining laminated composite materials, the risk is present of the layers separating (delaminating) due to the low interlaminar strength. Extensive delamination damage can reduce the strength of the material.
Low Heat-and-cold Resistance of Certain Thermoplastics.
The heat release during machining can cause the matrix to soften and to block the tool, making further machining impossible. In order to achieve good hole quality, therefore, effective cooling of the tool/hole edge is required, and the cut material (shavings, chips and grinding dust) must be removed continuously from the hole.
High Resistance to Wear of Fibres.
When composite fibres are machined by a process involving cutting, severe tool wear occurs as a consequence of the high wear-resistant characteristics of the fibre materials. This leads to high wear costs, especially when making holes with high precision requirements.
The methods used, to make holes in composite laminates traditionally involve drilling, milling, sawing and grinding. The problem associated with these hole forming methods as they are applied at the present time is that they are not sufficiently effective from a technicali/financial point of view.
One general problem associated with cutting machining with traditional metal-based tools is the high wear costs. Drilling calls for considerable care to be taken in order to avoid delamination damage on both the entry side and the exit side. In order to achieve the required hole quality, cutters made of special materials are needed, and special procedures must be worked out. In order to avoid extensive delamination damage to the exit side of the laminate, it is necessary for local lateral pressure to be applied around the edge of the hole. Another previously disclosed method of protecting the exit side from damage is to provide the laminate with an extra protective layer.
Sawing is a manifestly unsuitable method for producing holes with high accuracy requirements. Making holes by grinding involves the use of a hollow tubular body in the form of a cylinder, the cutting end of which is coated with a wear-resistant surface layer. Holes are produced by grinding the surface of the material transversally by causing the grinding tool to rotate. This method is slow and provides low precision.